1. Field of the Invention
The present invention relates to an optical fiber. Particularly, the present invention relates to an optical fiber in which an effective cross sectional area Aeff is enlarged so as to use for a long-haul wavelength multiple transmission system, and dispersion slope is restrained.
2. Description of Related Art
Transmission capacity for an optical fiber has increased drastically by the use of a Wavelength Division Multiplexing (hereinafter called WDM) method. In the WDM method, it is required that non-linear effect should be reduced and the chromatic dispersion should be controlled in an optical fiber, which is used for data transmission. For these purpose, the development for various optical fiber such as an optical fiber of which effective cross section a1 area Aeff is enlarged, an optical fiber of which dispersion slope is reduced, and an optical fiber for compensating the dispersion slope, have been made.
In order to increase the transmission capacity of an optical fiber by the WDM method, there are, in general, two ways of approach. One is a method by which the number of the wavelengths to be multiplexed is increased. Another approach is that the transmission speed is increased.
For a method for increasing the number of the wavelengths to be multiplexed, an approach is proposed by which the wavelength bandwidth to be used for data transmission is enlarged. In FIG. 9A, chromatic dispersion characteristics of a common WDM optical fiber (a) is shown.
For an WDM optical fiber, 1550 nm wavelength bandwidth is commonly used. In particular, a wavelength bandwidth which is called Conventional Band (1530 to 1565 nm, hereinafter called C-Band) has been used commonly. However, recently, there are proposed approaches in which wavelengths such as Long wavelength Band (hereinafter called L-Band) in 1565 to 1625 nm and Short wavelength Band (hereinafter called S-Band) in 1460 to 1530 nm should be used for data transmission.
For example, an optical fiber shown in FIG. 9B is proposed in which the chromatic dispersion is enlarged so as to be used in S-Band, C-Band, and L-Band.
In such optical fibers, there are various kinds of refractive index difference. In FIGS. 10 to 12, examples for such optical fibers are shown.
Among these optical fibers, an optical fiber having single-peak refractive index profile shown in FIG. 10 is used for a single mode fiber in an ordinary 1.3 μm bandwidth. Also, an optical fiber shown in FIG. 11 having stepped refractive index profile in which refractive indexes in two layers are different, and an optical fiber shown in FIGS. 12A and 12B having a segment refractive index profile in which a high refractive index section is in the center part and a plurality of layers having different refractive indexes are disposed are commonly used for a zero dispersion shift fiber in the 1.55 μm bandwidth and a non-zero dispersion shift fiber (hereinafter called NZ-DSF) which has been developed for WDM data transmission.
For an optical fiber which is used for WDM data transfer, following characteristics are required.
First, it is necessary to restrict a non-linear effect. Such a purpose can be realized by enlarging an effective cross sectional area Aeff. Second, it is necessary to restrict an occurrence of four-photon-mixture. Such a purpose can be realized by obtaining a local dispersion. Third, it is necessary to restrict a linear distortion caused by chromatic dispersion. Such a purpose can be realized by reducing an average chromatic dispersion over an optical path. Lastly, in order to control dispersion in wide range bandwidth, it can be realized by reducing the dispersion slope.
The above NZ-DSF was designed so as to be satisfied the required characteristics. However, it is very difficult to enlarge the Aeff and reduce the dispersion slope simultaneously. From a commercial point of view, an optical fiber (Aeff enlarged NZ-DSF) in which Aeff is enlarged to 70 μm2 and the dispersion slope is 0.09 ps/nm2/km, and an optical fiber (low dispersion slope optical fiber) in which Aeff is 50 μm2 and the dispersion slope is reduced to 0.05 ps/nm2/km, are commonly used for NZ-DSFs.
For an optical fiber which can realize more enlarged effective refractive index Aeff and more reduced dispersion slope, an optical fiber shown in FIG. 13 having a ring-shape refractive index profile in which a central part of the core has low refractive index and high refractive index section therearound are disposed is known. For an optical fiber having such a ring-shaped refractive index profile, an optical fiber in which Aeff enlarged to 100 μm2 and the dispersion slope is restricted at 0.06 to 0.08 ps/nm2/km is reported in “ECOC '96 MoB. 3. 2”.
However, there has not been a report that an optical fiber having the ring-shape refractive index profile can realize lower loss. For example, in the above report, 0.23 dB/km of loss has been reported. No other report mentions such lower loss characteristics. Therefore, it is necessary to reduce the transmission loss so as to realize an optical fiber having a ring-shaped profile.
In an optical fiber having conventional ring-shaped refractive index profile, an absolute value for a chromatic dispersion in 1550 nm wavelength is set to zero or lower than 6 ps/nm/km. Chromatic dispersion such as 6 ps/nm/km is in accordance with a specification of a conventional NZ-DSF. The refractive index profile of an optical fiber having such a chromatic dispersion area is shown in FIG. 14. Characteristics of this optical fiber is shown in TABLE 1.
TABLE 1TRANSMISSION LOSS[dB/km]0.231CABLE CUTOFF WAVELENGTHλ cc[nm]1440EFFECTIVE CORE CROSSAeff[μm2]93.9SECTIONAL AREAMODE FIELD DIAMETERMFD[μm]8.92CHROMATIC DISPERSION[ps/nm/km]5.0DISPERSION SLOPE[ps/nm2/km]0.071BENDING LOSS @ 20 φ[dB/m]7.4POLARIZATION MODEPMD[ps/√{square root over (km)}]0.08DISPERSION*Above values are measured under condition of 1550 nm wavelength
In FIG. 14, a horizontal axis indicates a distance from a center of the core in radial direction. A vertical axis indicates a refractive index difference between each core and the clad. Also, in FIG. 14, a product of a square of electric field E in a dominant mode which transmits in an optical fiber and the distance r in the radial direction is shown by dotted line. Also, in FIG. 15A, loss wavelength characteristics for wavelength λ is shown. In FIG. 15B, loss value under condition that the unit of the horizontal axis is 1/λ4.
The transmission loss of this optical fiber in the 1550 nm wavelength is 0.231 dB/km. This transmission loss is as high as that of the optical fiber having conventional ring-shaped refractive index profile. Also, value “a” which indicates an inclination to 1/λ4 is 1.183. Such a value is very large in contrast that a value “a” in an ordinary NZ-DSF is nearly 1.06. Such a high transmission loss in the optical fiber is caused by Rayleigh loss. In order to realize an optical fiber having low loss, it is necessary to reduce the Rayleigh loss.